This invention relates to a waveguide containing layered particles having lateral and width dimensions each of less than 1 micrometer and to a method of making the same, and a display screen employing same.
Optical screens typically use cathode ray tubes (CRTs) for projecting images onto the screen. The standard screen has a width to height ratio of 4:3 with 525 vertical lines of resolution. An electron beam is scanned both horizontally and vertically across the screen to form a number of pixels which collectively form the image.
Conventional cathode ray tubes have a practical limit in size, and are relatively deep to accommodate the required electron gun. Larger screens are available which typically include various forms of image projection. However, such screens have various viewing shortcomings including limited viewing angle, resolution, brightness, and contrast, and such screens are typically relatively cumbersome in weight and shape. Furthermore, it is desirable for screens of any size to appear black in order to improve viewing contrast. However, it is impossible for direct view CRTs to actually be black because they utilize phosphors to form images, and those phosphors are non-black.
Optical panels used for viewing images may be made by stacking waveguides. Such a panel may be thin in its depth compared to its height and width, and the cladding of the waveguides may be made black to increase the black surface area. It is known in the art that waveguide components are utilized for transmission of light. It is further known in the art that a waveguide has a central transparent core that is clad with a second material of a lower refractive index. In order to provide total internal reflection of light within this waveguide, the central core has a higher refractive index of refraction than the clad. By adjusting the difference in refractive index the acceptance angle of incoming light may be varied. The larger the difference in refractive index, the larger the incoming light acceptance angle.
However, optical waveguides of the step index cladding type have some significant drawbacks. In the formation of a large optical panel using stepped index clad waveguides many layers are stacked on top of each other and adhered to each other. In a typical 50xe2x80x3 diagonal screen there may be several hundreds or even thousands of waveguides that are adhered to one another. Handling and cutting many strips of thin polymer is very difficult. The compatibility of materials that have a refractive index difference from core to clad is limited. This may contribute to problems such as inadequate adhesion between layers. Such an incompatible may result in layer to layer interface problems such as air gaps or rough surface or layer separation. These types of problems may cause a loss of light at each bounce at the interface between the core layer and surrounding cladding layers. Although the loss of light at each bounce within the optical waveguide may be small, a light ray may undergo a large number of bounces as it traverses the core layer. Therefore, the amount of light loss that occurs in optical panels becomes a significant detriment to the overall efficiency and performance of the optical panel, as well as the quality such as brightness, and sharpness of the image.
Since there are a limited number of materials that can be used in combination between the core and clad that provide the desired delta refractive index, provide adequate adhesion between the layers and are capable of absorbing ambient room light, it is important to have a means of controlling or modifying the refractive index of polymers to assure that both optical and physical characteristics are optimized. In stepped refractive index clad waveguides of the type described in U.S. Pat. Nos. 6,002,826, 6,301,417 and 6,307,995, it is important to control or be able to modify the refractive index difference between two different materials or to modify the refractive index of a given material. If the difference is too large, the ambient light acceptance of the screen becomes large and does not appear to be black. There remains a need for improved control of refractive index as well as a broader selection of materials that can be used.
Ever since the seminal work conducted at Toyota Central Research Laboratories, polymer-clay nanocomposites have generated a lot of interest across industry. The utility of inorganic nanoparticles as additives to enhance polymer performance has been well established. Over the last decade or so, there has been an increased interest in academic and industrial sectors towards the use of inorganic nanoparticles as property enhancing additives. The unique physical properties of these nanocomposites have been explored by such varied industrial sectors as the automotive industry, the packaging industry, and plastics manufactures. These properties include improved mechanical properties, such as elastic modulus and tensile strength, thermal properties such as coefficient of linear thermal expansion and heat distortion temperature, barrier properties, such as oxygen and water vapor transmission rate, flammability resistance, ablation performance, solvent uptake, etc. Some of the related prior art is illustrated in U.S. Pat. Nos. 4,739,007; 4,810,734; 4,894,411; 5,102,948; 5,164,440; 5,164,460; 5,248,720; 5,854,326; and 6,034,163.
In general, the physical property enhancements for these nanocomposites are achieved with less than 20 vol. % addition, and usually less than 10 vol. % addition of the inorganic phase, which is typically clay or organically modified clay. Although these enhancements appear to be a general phenomenon related to the nanoscale dispersion of the inorganic phase, the degree of property enhancement is not universal for all polymers. It has been postulated that the property enhancement is very much dependent on the morphology and degree of dispersion of the inorganic phase in the polymeric matrix.
The clays in the polymer-clay nanocomposites are ideally thought to have three structures: (1) clay tactoids wherein the clay particles are in face-to-face aggregation with no organics inserted within the clay lattice; (2) intercalated clay wherein the clay lattice has been expanded to a thermodynamically defined equilibrium spacing due to the insertion of individual polymer chains, yet maintaining a long range order in the lattice; and (3) exfoliated clay wherein singular clay platelets are randomly suspended in the polymer, resulting from extensive penetration of the polymer into the clay lattice and its subsequent delamination. The greatest property enhancements of the polymer-clay nanocomposites are expected with the latter two structures mentioned herein above. Most of the work with nanoclays has been for physical properties modification.
There is a continuing need for improved waveguides that have a broader selection of materials and where the difference in refractive index can be controlled more finely in a given material to optimize the incoming light acceptance from the light projector while minimizing the ambient light acceptance on the viewing side of the screen.
The invention provides a waveguide comprising a transparent polymeric central core clad externally with one or more polymeric layers, at least one of the core or clad layers containing layered particles disposed in a polymeric binder, wherein a majority of the particles have a lateral dimension less than 1 micrometer.
The invention provides a means to control or modify the refractive index of many polymers that when used in a stepped clad waveguide and enables a broader selection of materials that can be adhesively joined.